Graphene is the name given to a two dimensional (2D) monolayer of carbon atoms, in which the atoms are bonded to each other in a 2D hexagonal lattice structure. Graphene can be considered to be the basic building block of various other forms of carbon: graphite consists of layers of graphene stacked to form a three dimensional (3D) material, carbon nanotubes are commonly described as rolled-up sheets of graphene, and fullerenes are nanometer-sized balls of graphene.
Carbon nanostructures have been proposed for use in many different applications, particularly in nanotechnology and materials science. Carbon nanotubes and fullerenes have been used for various applications, but their electric, magnetic and elastic properties all originate from the parent structure. However, graphene has yet to receive the attention that has been thrust upon both nanotubes and fullerenes, due to the problem of making it in bulk quantities for realistic applications. Both fullerenes and carbon nanotubes can be grown by a number of different methods that can be utilized for continuous synthesis. However, existing methods for creating graphene are not viable for industrial or mass production.
Much has been made of the properties of this material, especially its electronic properties. Electrons in graphene behave like relativistic particles that have no rest mass and travel at 106 meters per second. This value is 300 times slower than the speed of light in a vacuum but is much faster than electrons travelling in an ordinary conducting material. Graphene exhibits a room temperature quantum Hall effect, and an ambipolar electric field effect, as well as the ballistic conduction of charge carriers. Graphene is a material suggested as a solution to the problem of transistors based on a silicon oxide gate. Unlike all other known materials, graphene remains highly stable and conductive even when it is cut into devices one nanometer wide. Graphene transistors begin to show advantages and good performance at thicknesses below 10 nanometers—the miniaturization limit at which current silicon technology is expected to fail. A graphene layer has also been demonstrated as a conducting transparent electrode in a device.
Mechanical investigations into the structure of graphene sheets using an AFM tip gave a Young's modulus of 0.5 TPa (200 times stronger than steel on a nanoscale level). This high value suggests that graphene has a high strength and rigidity giving it application in the field of nanoelectromechanical systems (NEMS) such as pressure sensors and resonators, as well as application to fillers in nanocomposites.
Many of the early investigations into graphene and its properties were conducted on samples that had been mechanically cleaved from bulk graphite, often using adhesive tapes or micromechanical cleavage. This process is difficult to scale up, and suffers from the problem that the small amounts of graphene produced are hidden in large quantities of thin graphite flakes. Alternatively, oxidation of bulk graphite to graphene oxide interrupts the interactions between the layers allowing them to separate when dispersed in a solvent. Like the oxidation of carbon nanotubes, the process has a detrimental effect on the properties by introducing large defects into the structure. To regain the graphene structure, an extra step such as thermal annealing in an inert atmosphere or reduction using hydrazine is required. Disruption of the layers of graphite to produce graphene can also be achieved using liquid phase exfoliation using solvents and surfactants, but again suffers from poor yields and the need to use large volumes of solvent or surfactant.
More recent research has been focused on synthetic production of graphene from other carbon sources. Graphene has been synthesized by pyrolysis of betaine and camphor (over nickel metal), silicon carbide reduction and ethanol via microwave irradiation. The decomposition of hydrocarbons over metal substrate has also been known to produce some graphene. Also recently, carbon nanotubes have been reported as a source of graphene by selective oxidation or “longitudinal cutting” of the cylindrical wall with potassium permanganate and sulphuric acid or ionized argon gas. However, each of these methods suffer from some common drawbacks: (i) low yield and synthesis of other carbon morphologies during the procedure, which limits extensive studies and development on the material; (ii) the thickness of the material that is produced is rarely below 10 nanometers; and (iii) they often require a sophisticated apparatus (microwave and high pressure reactors), controlled atmosphere, high temperature (silicon carbide reduction requires 1500-2000 degrees Celsius), time-consuming steps, transition metal catalysts, or highly flammable and potentially explosive gaseous mixtures.
Commercial production of graphene for the global market is currently based on the micromechanical cleaved and oxygen intercalation methods, both of which are time consuming, with the latter still containing a large amount oxidized graphene with inferior electrical and mechanical properties. The high production costs involved are reflected in the high market price.
WO 2009/029984 A1 describes a process for producing graphene wherein an alkali metal is reacted with an alcohol to produce a solvothermal product comprising a metal alkoxide. The solvothermal product is then pyrolysed to produce the graphene. A drawback of this process is that the reaction to form the solvothermal product, on which the process relies, is a lengthy process, taking about 72 hours. Furthermore, both the production of the solvothermal product and the pyrolysis step are necessarily performed as a batch process. As a result, the process is not well-suited to industrial-scale manufacture. Equally, formation of the solvothermal product generates high pressures in excess of 100 bar and the addition of sodium metal to a small amount of alcohol is a very exothermic reaction generating a lot of heat and the explosive gas hydrogen. As the pyrolysis step requires the presence of oxygen, the graphene produced may be oxidised to graphene oxide, again lowering the yield of graphene.